When a product is packaged at sea level and is subsequently transported to the mountain, the external pressure decreases. Due to that reason packages may even burst as a consequence of the internal-external pressure difference. Such was the case of the “Pringles” potato chips, which sale was discontinued in those countries with great altitude differences, as packages lids were expelled during their transport, exposing the contents thereof. There are also packaged products, such as coffee, which release gases generating an over-pressure capable of bursting the package.
One-way valves are thence a valid alternative by which this kind of problems may be solved.
Packages including a one-way valve may be made of a rigid material or a flexible laminar material, said material being totally impervious all over its surface, said surface exhibiting a plurality of small orifices on a small portion thereof. The purpose of said orifices is to allow gas produced within the package to migrate towards the environment as a result of the generated over-pressure or simply in order to equalize internal-external pressures when said package is transported.
In order to prevent entering of environmental oxygen into the package, said orifices are covered by a one-way valve, which is in closed position when the package internal pressure is equal or lower than the environmental pressure, and in open position when the internal pressure is higher than the environmental one (over-pressure).
A particular application of this kind of valves involves roasted coffee. Roasted coffee, once packaged, retains its aroma for about 8-10 weeks. After such time, there begins an ageing period as a result of catalyzed oxygen condensation and polymerization reactions, which culminate with the formation of small quantities of peroxides which adversely affect coffee's aroma and flavor.
Many intents have been made with the object of developing coffee packages bearing a minimum quantity of oxygen, such as vacuum packages. Nevertheless, this type of packages undergoes a slow production of gases, such as carbon dioxide, which inflate same and thence there exists the risk of packages bursting upon their stacking. Such gases production within the packages is due to the fact that during the roasting process, along with coffee's characteristic brown colour and aroma development, a large quantity of carbon dioxide slowly releases from coffee grains during the first three weeks, thus producing package swelling.
In order to solve this problem one-way valves have been developed, application thereof not being limited to coffee packages but may be also useful for any package bearing products capable of generating gases or which are to be transported through areas of wide pressure differences. The important fact regarding these valves is that they should open upon the most minimum over-pressure, allowing gases to be expelled from the package towards the external environment and at the same time they should be able to close as fast as possible in order to prevent external oxygen from entering the package and thus give rise to an undesirable oxidation.
One-way valves are widely known, and their structure may be divided in two classes: flexible or rigid. Some documents which disclose the later are U.S. Pat. Nos. 2,870,954, 3,717,276, 4,051,973, 4,122,993, 4,000,846, 5,584,409, DE 4435492 A1, DE 2716299, DE 3031208 A1 and EP 24310 A1. The problem with rigid valves, however, is that they are not very adaptable to flexible packages. As they are not able to meet mechanical solicitations to which packages are subject during their transport and storage, there exists a significant risk of detachment of said valves. This could cause spilling and/or contamination of the packaged product.
On the other hand, packaging speed is lower when using rigid valves, as they must be applied to the package on the packaging line, application times being longer than those required in the case of flexible valves. The advantage of flexible valves is that they may be applied to packages before the packaging process takes place.
As regards flexible valves, there can be mentioned the following prior art documents.
Patent U.S. Pat. No. 2,821,338 (Metzger) discloses a one-way valve consisting of a pair of elongated and overlapping sheets: a top one contacting the external environment and a bottom one contacting the package film and bearing an opening, preferably a circular one, which coincides with and exhibits a larger diameter than said package opening. Both sheets are comparatively thin, they are made of the same flexible, impervious material, and they are joined by two of their sides by means of a thermoseal. A small strip, also made of flexible and impervious material, is arranged between said top and bottom sheets, more specifically over the opening of the bottom sheet. This small strip acts as an auxiliary “flap” type valve and also as isolation for said opening. As this small strip material is not thermo-sealable it prevents the valve top sheet portion, which overlaps the circular opening, from sealing against the bottom sheet during the valve sealing, which fact would hinder the operation thereof.
U.S. Pat. No. 2,927,722 (Metzger) discloses a one-way valve which is similar to the above, characterized in that, in order to assist in the valve aperture should there exist a high cohesion between overlapping sheets, a small area of the adhesive coating which joins the top and bottom sheets one to the other and to the auxiliary “flap” type valve, it is rendered non-adhesive by means of the inclusion of a substance such as talc, glycerine or any other class of similar inert non-adhesive substance. Said small area is preferably located near one of the edges of the valve.
According to U.S. Pat. No. 2,946,502 (Metzger) said inert non-adhesive substance extends over the full surface of the valve central portion including the opening, thus generating an adhesive-free path for gases to be expelled.
Document GB 1169280 discloses a plastic bag provided with a perforated surface on one of its walls, a flexible plastic perforated sheet fixed to said wall and covering said perforated surface, and an air-permeable material barrier sandwiched between said perforated surface and said sheet. Such permeable material consists of an elastic sponge with interconnected rubber cells or a synthetic plastic material; such perforated sheet being fixed to said bag wall by means of a thermoseal.
Patent EP 23703 B1 discloses a valve which works even under stacked bags. To such effect and on the opposite side of the valve where the adhesive coating is applied in order to attach same to the package, there are applied spacing strips which are thicker than the valve, preferably 200 micrometers thick, whereas thickness of sheets which conform the valve itself is not greater than 35 micrometers.
Patent DE 2331862 discloses a valve which allows gases from the interior of a package to be released through a series of orifices made on the top sheet thereof, which orifices do not coincide with the package opening.
Documents DE 3526586 A1 and DE 3618829 disclose over-pressure valves conformed by a single flexible membrane which is fixed by means of an adhesive to a package wall, preferably to a base thereof. Said membrane blocks a passage orifice made on said package and defines an outlet channel for gases. In order to prevent environmental air diffusion through said channel, a liquid blocking means, such as silicone oil, is introduced therein. Thus, said channel acts as a filter, more particularly a wet filter.
U.S. Pat. No. 4,134,535 describes a valve with one or two sheets which uses a porous element bearing a highly cohesive non-volatile liquid or respectively high surface tension which is enclosed between the package wall and one of the sheets or between said two sheets. The top sheet exhibits orifices through which gas from the interior of the package is released due to the capillarity effect. Upon a certain over-pressure from the interior of the package cohesiveness or high surface tension of the liquid within the porous element is broken, thus allowing gas passage through it towards the outlet orifices. Forces originated from the over-pressure are higher than cohesion forces between liquid particles. After pressures are equalized, capillarities formed in the liquid are closed again by surface forces.
U.S. Pat. No. 5,263,777 discloses a one-sheet valve adhesively attached by two of the laterals thereof to one of the walls of the package onto which it is applied, covering the different orifices existing on the later for the interior-originated gases to be expelled. The adhesively attached region adopts the form of wedged adhesive strips in order to allow the correct functioning of the valve when packages are stacked. Such wedge operates in a manner similar to spacing strips of above referenced patent EP 23703 B1.
U.S. Pat. No. 5,782,266 includes an alarm device incorporated into a pressure-relief valve in order to protect the package contents. Said device adopts the form of a strip which is adhesively attached to the valve through one of its sides.
All of the above flexible valves have drawbacks. For instance, when using a high cohesion and/or surface tension fluid medium, such as oil, in order to block the passage of environmental oxygen into the package, surface tension of said fluid medium placed between the two overlapping sheets may be so high that cohesion could prevent over-pressure of the package from detaching the top sheet from the bottom one, thus rendering the valve inoperable, even leading to packages bursting. On the other hand, when a granular medium is used in order to assist in the valve aperture, as for example talc, particles could cover the gases outlet opening, or even the valve could remain permanently opened.
Consequently, valves bearing a fluid, granular or gas medium in inter-laminar spaces have drawbacks which hinder the operation thereof.